Put it to sleep... Never heard it put that way but you can force stop the app with your app manager from your settings screen or you can disable from that same location. Force stopping it will allow it to open again if it's one that opens on its own. If it's one that opens on its own any kind of app killer or battery saver will likely cause more of a strain on your device trying to close it every time it wants to open. You're better off disabling it and turning out on only when needed. You may also want to check the settings for the app. There maybe something there that limits this behavior.

What is the app? Specifically naming the app may yield better less general responses.